This invention relates to an improvement in making polyethylene terephthalate useful in molding containers such as bottles. More specifically, this invention relates to an improved process for producing polyethylene terephthalate which is good for making bottles for carbonated beverages.
The hazards of using glass containers for beer or carbonated beverages are well known. Dangerous breakage often takes place due to the internal pressure exerted by the pressurized gas in the bottles as well as by dropping or otherwise impacting the bottles, not only in the course of production and distribution of the bottled product, but also in handling of the bottled product by consumers.
Recently, the carbonated beverage industry has begun to use plastic, rather than glass, bottles for their beverages. Besides avoiding the hazards of breakage, plastic bottles also have the advantage of being much lighter than glass. Furthermore, less energy is required to make and transport plastic bottles.
Polyethylene terephthalate (hereinafter "PET") is a polymer which is particularly well suited for such applications. PET may be prepared, as is well known, by the esterification of ethylene glycol and terephthalic acid or by the ester interchange of dimethyl terephthalate with ethylene glycol, followed by condensation polymerization in the presence of a catalyst such as antimony trioxide, e.g., at a temperature of about 285.degree. C. and at an absolute pressur of about 1 millimeter of mercury. The PET product may then be extruded and pelletized. Unfortunately, these PET pellets cannot be used directly in the preparation of carbonated beverage bottles, because excessively large amounts of acetaldehyde are produced in the pellets under the conditions by which they are normally made. Furthermore, even if all of this residual acetaldehyde were removed from the PET pellets, it has been found that additional acetaldehyde is generated when a preform is molded from the PET, the molding occurring at temperatures generally above 250.degree. C. The presence of acetaldehyde usually affects the taste of any carbonated beverage, but particularly a cola flavored beverage which might be placed in such a container.
Accordingly, a commercially acceptable PET bottle must be prepared from PET which has only insignificant amounts of acetaldehyde present and which furthermore does not generate significant additional amounts of acetaldehyde when heated for molding into a container or bottle. Stringent standards are imposed by molders producing beverage bottles. In addition, the PET which is used in making such containers should have a suitably high intrinsic viscosity, i.e., above about 0.60, preferably 0.65 to 0.07, deciliters per gram as calculated from measurements made on an eight percent solution in o-chlorophenol at 25.degree. C.